The present invention relates to a glass-ceramic plate, intended in particular to cover heating elements.
More particularly, the invention is directed toward a glass-ceramic plate used as a cooking top. Such a cooking top is provided in particular with underlying heating elements such as heat sources of halogen-lamp or radiant type.
To be usable as a cooking top with underlying heating elements, a glass-ceramic plate must meet a certain number of criteria:
on the one hand its transmission of wavelengths in the visible region must be sufficiently low to mask at least part of the underlying heating element(s) to ensure that the user is not dazzled by such underlying heating element(s) when in operating condition and, from the viewpoint of safety, must also be sufficiently high that the user can visually recognize that the heating element(s) is or are in operating condition;
on the other hand, in order to optimize heating and cooking efficiency, its transmission of wavelengths in the infrared region must be high.
On the present market for glass ceramics there exist two types of plates which can be differentiated by their appearance: one plate of transparent brown appearance and one plate of opaque white appearance.
The first type mentioned hereinabove is used when the associated underlying elements are heating elements such as heating sources of the halogen-lamp or radiant type. The second type mentioned hereinabove is associated exclusively with induction-heating means.
The present market demand is oriented toward new appearances of glass-ceramic plates, particularly toward appearances in harmony with the kitchen furniture.
The invention attempts to respond to current market demand and to propose a new type of glass-ceramic plate whose appearance and optical characteristics are still compatible with a use of underlying heating elements such as elements of the radiant or halogen-lamp type.
Consequently, the object of the invention is a glass-ceramic plate based on silicon, aluminum and lithium oxides, intended in particular to cover heating elements. According to the invention, said plate has a diffuse transmission factor of at least 50%. It is pointed out that the diffuse transmission factor is a measure of light diffusion and, in the scope of the invention, is defined as the ratio of diffuse transmission to total transmission at a wavelength of 550 nm.
The diffuse transmission factor is preferably greater than 70% and advantageously greater than 90%.
According to an advantageous characteristic, the plate according to the invention exhibits light transmission TL of between 5 and 40%, especially of between 5 and 10%, preferably of between 6 and 9%.
The glass-ceramic plate proposed in this form by the invention overcomes the cited technical problem.
According to another characteristic, the plate according to the invention has as its preferably single crystalline phase a solid solution of xcex2-spodumene crystals, in which the size of the individual crystallites is advantageously smaller than 100 nm.
Advantageously, the plate according to the invention has a coefficient of linear expansion smaller than 15xc3x9710xe2x88x927/xc2x0 C., especially equal to 9xc3x9710xe2x88x927/xc2x0 C. in the temperature range of between 20xc2x0 C. and 700xc2x0 C.
The plate defined hereinabove is also remarkable in that it is composed substantially of the following oxides, expressed in wt %:
The invention is also directed toward a process for manufacture of a glass-ceramic plate based on silicon, aluminum and lithium oxides, intended in particular to cover heating elements, in which process there is performed at least one cycle of ceramization of a glass plate, the said ceramization cycle including a crystallization plateau corresponding to a duration t at a temperature T for formation of the xcex2-quartz crystalline phase.
According to the process of the invention, there is performed a heat-treatment cycle in which the duration txe2x80x2 is longer and/or the temperature Txe2x80x2 is higher by at least 5% compared with the duration t and/or temperature T respectively.
According to a first variant of the invention, the difference Txe2x80x2xe2x88x92T is at least 40xc2x0 C., preferably 100xc2x0 C.
According to a second variant of the invention, the difference txe2x80x2xe2x88x92t is at least 15 minutes.
Finally, the invention relates to a glass composition used for operation of the process cited hereinabove, which composition is remarkable in that it comprises the following oxides, expressed in wt %:
Within the scope of the invention, when such a glass composition is used, the resulting plate has a substantially xe2x80x9cmilkyxe2x80x9d appearance. obviously all kinds of coloring oxides can be added to the glass composition without modifying the desired effect, which is a diffusing appearance.
In fact, the authors of the present invention have found unexpectedly that this diffusing appearance of the glass-ceramic plate is due to the presence of xcex2-spodumene crystals as the single crystalline phase.
Heretofore the person skilled in the art would have assumed that such a phase would necessarily impart an opaque appearance to a glass-ceramic plate.
The glass-ceramic plate proposed by the invention can be used in a device for cooking and/or holding at high temperature other than that targeted in particular by the invention, such other device being provided with heating means other than heating elements of the radiant or halogen-lamp type, examples being induction and/or gas heating means.
Of course, the glass-ceramic plate according to the invention can also be used in a hybrid cooking device, or in other words one comprising at least two different heat sources of direct and indirect heat-transfer type, for example at least one heating element such as a radiant heater, a halogen lamp and at least one atmospheric gas burner and/or at least one induction-heating means.
In this variant, the plate is then provided with at least one zone intended to cover the heating element and at least one opening intended to accommodate the atmospheric gas burner and/or at least one zone intended to cover the induction-heating means.
It can be provided that this opening be shaped and that it be disposed at the top of a local deformation of the plate such as described in French Patent Application 97-061114 filed on May 20, 1997.
Other advantageous details and characteristics will become clear hereinafter from the description of non-limitative practical examples of a glass-ceramic plate according to the invention.
It is pointed out first of all that the glass-ceramic plates according to the invention and corresponding to Examples 1 to 5 all have the same composition.
This comprises the following oxides, expressed in wt %:
Glasses having the composition mentioned hereinabove are melted at around 1650xc2x0 C. in such a quantity that there can be rolled a continuous glass sheet from which there are cut glass plates having final dimensions of 56.5 cmxc3x9756.5 cmxc3x970.4 cm.
These glass plates are usually ceramized in a manner known in itself on ceramic screens in a ceramization cycle. This ceramization cycle comprises in known manner the following stages:
a) elevation of the temperature at 50 to 80 degrees per minute up to the nucleation region, which is generally close to the glass transition region;
b) passage through the nucleation interval (670 to 800xc2x0 C.) in about twenty minutes;
c) elevation of the temperature in 15 to 30 minutes to the temperature T of the ceramization plateau, which lies between 900 and 960xc2x0 C.;
d) holding the temperature T of the ceramization plateau for a time t of 10 to 25 minutes;
e) rapid cooling to room temperature.
It is pointed out that, within the scope of the invention, a cycle of ceramization of a glass plate based on SiO2xe2x80x94Al2O3xe2x80x94Li2O is a heat-treatment cycle which includes a plateau, at the end of which the glass plate based on SiO2xe2x80x94Al2O3xe2x80x94Li2O contains the xcex2-quartz crystalline phase.